What are bacteria?

This colorized image (a scanning-electron micrograph) shows four spherical methicillin-resistant Staphylococcus aureus (MRSA) bacteria (purple) in the process of being ingested by a human neutrophil white blood cell (blue). (Image credit: Callista Images via Getty Images)
Bacteria is a single-celled organism that can be found in almost everything: the ground, the ocean, your hands, and your gut. Some bacteria can be harmful but most are harmless. Others are beneficial for human health. Many times, people live in symbiosis between bacteria and humans, maintaining mutually beneficial relationships without realizing it.

Let's now dissect this diverse group single-celled organisms. Here's a brief overview of bacteria, their functions and the ones you should be looking out for.

What is bacteria?

(Image credit: Shutterstock)

Bacteria is a single-celled organism with an unique internal structure. Eukaryotes are multicellular organisms that have distinct nuclei attached to a membrane. This is different from the eukaryotics found in humans and other multicellular organisms. Bacteria is prokaryotes. This means they don't have organized nuclei and other membrane-bound organelles.

Bacterial DNA is free to move within bacteria cells, in a thread-like, twisted mass called the nucleoid. Some cells also contain separate pieces of DNA, called plasmids. According to the Microbiology Society, plasmids can contain genes that confer a survival advantage on bacteria, such as genes that transmit antibiotic resistance.

Bacteria should not be confused with archaea, the major group of prokaryotes. Archaea can also be single-celled organisms. However, the types of molecules that they use to build cell walls and the metabolic processes they use are different.

Structure of bacteria

Lactobacillus acidophilus is a milk-curdling bacteria. They are cylindrical-shaped. (Image credit: Shutterstock)

There are five basic shapes for Bacteria: spherical (or cylindrical), comma-shaped (or corkscrew) and spiral. These shapes have the scientific names cocci (round), vibrios ("comma-shaped"), vibrios ("cylindrical"), spirochaetes (“corkscrew”) and spirilla („spiral”). Names often reflect the shapes and configurations that bacteria take. The Lactobacillus acidophilus bacteria, which is milk-curdling, is a type of bacilli. Streptococcus pneumoniae, which causes pneumonia, is a type of cocci.

Bacterial cells have an outer and inner cell wall. Some bacteria, such as the mycoplasmas have no cell wall. Some bacteria might even have an outermost protective layer called the capsule. Sometimes, whip-like extensions cover the bacteria's surfaces, either long ones called flagella or shorter ones called pili. They help bacteria move and attach to their host.

According to the Science Education Resource Center of Carleton College, bacteria can be classified based on the composition of their cell wall using a test called Gram stain. Gram-positive bacteria is bacteria that does not have an outer membrane. Gram-negative bacteria that have an outer membrane won't be stained. S. pneumoniae is an example of a Gram-positive bacteria, while Escherichiacoli can cause food poisoning and Vibriocholerae can cause cholera are Gram-negative bacteria.

Under the cell membrane and cell wall, bacteria contains cytoplasm. This is a mixture of water and salts. The cytoplasm contains the nucleoid and plasmids as well as tiny protein factories called "ribosomes", which are where genetic instructions of the cell are translated into products. Tetracycline and other antibiotics target bacteria ribosomes in order to stop them from synthesizing protein, thus ending the life of the cell.

Some bacteria's cytoplasm may contain small pockets called inclusions that store nutrients for when they are needed. The chromatophores are structures that photosynthetic bacteria use to generate energy from sunlight. They may be found throughout their cytoplasm. These chromatophores contain pigments that are used in photosynthesis.

How does bacteria reproduce and eat?

Bacteria is one of the oldest forms of life on Earth. They have developed a wide range of survival strategies. Some bacteria can be photosynthetic while others can decompose organic matter into nutrients. Some bacteria form symbiotic or mutually beneficial relationships with their host.

According to Cornell University College of Agriculture and Life Sciences, most bacteria reproduce by binary fission. A single bacterial cell (the "parent") makes a copy its DNA and becomes larger by increasing its cellular content. The cell splits, pushing out the duplicated material and creating two identical cells called "daughter".

Budding is a method of reproduction for some bacteria types, like cyanobacteria or firmicutes. The daughter cell is an offspring of the parent. It begins as a tiny nub and grows to the same size as its parent before splitting off.

The DNA in the offspring and parents is identical after binary fission. By incorporating additional DNA from their environment into their genomes, bacteria cells introduce variation to their genetic material. According to the San Diego State University College of Sciences, horizontal gene transfer is a form of gene transfer. Live Science reported that the resulting genetic variation allows bacteria to adapt and thrive in changing environments.

Horizontal gene transfer can occur in three ways: transformation, transduction, and conjugation.

This diagram shows the stages involved in bacterial conjugation. (Image credit: Shutterstock)

Transformation is the most common method of horizontal gene transfer. It occurs when a bacteria absorbs DNA fragments from the surrounding environment and then uses them to create its cell membrane. Other bacteria release the DNA fragments into the environment. A bacterium must be competent to undergo transformation. This happens when nutrients are low or the density of a bacterial population is high. It might be evolutionary advantageous to test out new DNA in these situations.

Transduction is when a virus takes DNA from one bacterium to infect another, inserting a new gene sequence. Conjunction occurs when bacteria come into direct contact. A recipient cell receives DNA directly from a donor cell by forming a tube-like appendage called a pilus. This is what happens in E.coli, where some cells have a particular type of plasmid called the fertility factor or F factor. According to Modern Genetic Analysis (W. H. Freeman and Company 1999), this occurs. F factor-negative cells can be given DNA by F factor factor cells. The third type of transfer, called conjugation, aids in the spread of antibiotic-resistance genes.

What is the health benefit of bacteria?

Many bacteria can be beneficial to us. They can turn milk into yogurt or ferment cabbage into Kimchi. Some species do their work within us. According to the Microbiology Society there are approximately 10 times more bacteria cells than human cells in a person's body. Many of these cells live in the digestive system. These bacteria receive a steady stream of nutrients from our gut. They also help to break down foods that the human digestive enzymes cannot. Bacteroides taiotaomicron is an example of a bacteria that helps to break down complex carbohydrates. According to Mount Sinai's health library, L. acidophilus is responsible for breaking down sugars in milk. It also creates byproducts like lactic acid or hydrogen peroxide. These byproducts make it less friendly to harmful bacteria.

According to Nature Reviews Microbiology, bacteria on the skin can produce byproducts that protect against harmful bacteria. For example, the benign bacterium Corynebacterium is able to inhibit the growth of S. pneumoniae, which can cause pneumonia.

Some skin bacteria can be both beneficial and harmful. Staphylococcus epidermidis, a spherical bacteria that colonizes skin, can infect the body if it gets into the bloodstream. S. epidermidis produces proteins that can inhibit the growth its more dangerous cousin, Staphylococcus Aureus. S. aureus can also cause infections if it penetrates the skin barrier, but these infections are usually more severe than S. epidermidis.

What is the health impact of bacteria?

During a 1965 plague study, technicians in a San Francisco bacteriology lab isolate the bacteria Yersinia Pestis. (Image credit: Image courtesy CDC/Margaret A. Parsons. (Photo by Smith Collection/Gado/Getty Images)

S. aureus bacteria is a type of bacteria that can live with humans for most of the day. According to the Centers for Disease Control and Prevention, about 30% of people have S. aureus in the nose. These bacteria can cause serious infections if they make their way into the body, particularly in immunocompromised patients. According to the CDC, Staph infections can lead to sepsis (full body inflammation due to infection), pneumonia, endocarditis and osteomyelitis (inflammation and swelling of the bones).

Other bacteria can be very harmful to people. V. cholerae is responsible for Cholera which is a diarrheal illness that kills approximately 95,000 people each year. Black Death was caused by the bacterium Yersinia Pestis. It was spread by fleas which bite rodents. Bacillus anthracis is capable of forming anthrax spores in soil that can be inhaled or eaten.

People can be infected by some of the most problematic bacterias through spoiled foods. Salmonella bacteria can cause salmonellosis. This illness is characterized by stomach cramping, fever, diarrhea and stomach cramping. According to the CDC, although most people feel better within four to seven days of being infected, salmonellosis can cause serious illness and even death in older adults and children.

E.coli, another bacterium that causes food poisoning, is often spread through contaminated water and food. Many strains are harmless in the intestines of humans, but some can cause diarrheal illnesses. E.coli diarrhea, like salmonellosis is, is usually very unpleasant and short-lived. However, 5% to 10% of people who experience hemolytic uremic syndrome (a kidney complication) can become life-threatening according to the CDC.

Helicobacter Pylori is another common bacterium that could be dangerous to humans. According to the Mayo Clinic, about half of people have this bacteria in their stomachs. Although most people don't experience any symptoms, some people may develop peptic ulcers, which are painful sores that affect the stomach lining. Although it is not clear how bacteria spreads, risk factors include crowded living spaces.

What is bacterial vignanosis?

Bacterial Vaginosis refers to a condition where anaerobic bacteria (bacteria which do not use oxygen in metabolism) takes over Lactobacillus, a beneficial bacteria in the vagina. According to the Mayo Clinic, symptoms include vaginal itching and gray or green urine.

Bacterial Vaginosis is quite common. A nationwide study between 2001-2004 found that 29% had been randomly tested for the bacteria that causes the condition. This corresponds to approximately 21 million women affected in the U.S. at any one time. Only 15% of women who were positive for the bacteria had symptoms.

According to the Mayo Clinic, it is not known what causes bacterial vaginal infections. Some people may be more susceptible than others because their vaginal environment doesn't allow for healthy Lactobacillus bacteria. Doing sex with a partner, or multiple partners, can increase the risk of developing bacterial vaginosis. This is because they disrupt the normal bacterial communities in the vagina. According to the CDC, bacterial vaginosis is often caused by Gardnerella vaginalis and Prevotella species as well as Mobiluncus and Mobiluncus bacteria.

Untreated, bacterial vignaisis can lead to preterm births and increase the risk of sexually transmitted diseases. Antibiotics can be used to treat bacterial vaginosis.

Antibiotic resistance

The image of spherical bacteria is shown by the artist. Both Streptococcus and Staphylococcus are spherical. (Image credit: Kateryna Kon/Shutterstock)

Antibiotics are commonly used to treat bacterial infection. In recent years, however, the inappropriate and unneeded use of antibiotics has led to several antibiotic-resistant strains.

The resistant bacteria is no longer able to withstand previous effective antibiotics. The CDC estimates that at least 2,000,000 Americans are infected each year with antibiotic-resistant bacteria, which results in t23,000 deaths annually.

Dr. Christopher Crnich is an infectious-disease physician, hospital epidemiologist and hospital epidemiologist at William S. Middleton Memorial Veterans Hospital. "Pretty much every infection you can now think of has been identified as being associated mit some level of resistance," he said. "There are very few infections we treat now where resistant bacteria is not a clinical concern."

MRSA is one of the most well-known antibiotic-resistant bacterial strains. It resists methicillin as well as other antibiotics that are used to treat Staphylococcus infection. This is mainly acquired through skin contact. MRSA can spread to the bloodstream and cause pneumonia in hospitals and nursing homes. MRSA can also spread to communities. According to the CDC, this is especially true in areas where there are a lot of exposed skin and other contact with the body. MRSA acquired in the community is most commonly responsible for severe skin infections.

Antibiotic resistance can be combated by being careful with their use. Crnich stated that it is crucial to use antibiotics wisely. You should only use antibiotics if you have a severe bacterial infection.

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This article was last updated by Stephanie Pappas, Live Science Contributor.